1. Field of the Invention
The present invention relates to an apparatus and method for filtering particulate matter from dielectric fluids, and in particular, for filtering particulate matter from parts-washing solutions and similar fluids.
2. Discussion of the Prior Art
Mechanical filters, comprising screens, meshes, or other porous barriers, have long been used to remove impurities from oil. An inherent problem with such mechanical filters is that they are only capable of removing (i.e. capturing) particulate matter which is larger than the openings in the filter. Particulate matter which is smaller than the openings in the filter simply pass through the openings in the filter and continue to contaminate the fluid. When used with most oils, the practical lower limit to the size of filter openings typically has been found to be five microns (0.0002 inches) or larger. In order to remove smaller particles, the filter media must be so dense (i.e. the openings in the filter must be so small) and thick that extremely high pressures must be used to force the fluid through the filter. It has been found that in most parts-washing applications, most of the solid contaminants suspended in oils are smaller than five microns (0.0002 inches). In other words, the particulate sizes that are most plentiful in typical contaminated parts-washing fluids, and which ate capable of doing extensive damage to machinery, are smaller than the smallest openings in common mechanical filters and, accordingly, pass through such filters.
It is well known in the art to remove contaminants from parts-washing solvents, hydraulic fluids, and other dielectric fluids electrostatically. Electrostatic filters remove solid contaminants that are extremely small (down to sub-micron range) while not requiring high fluid pressures which would be necessary, for example, in mechanical filtering systems. Electrostatic filtration may also remove tars, varnishes, and some acids. Electrostatic fluid filters work by passing contaminated fluid over or through a plurality of alternately charged electrodes. The electrodes are alternately charged positive and negative, or are alternately energized by high voltage and grounded. In the prior art porous filter media are disposed intermediately between adjacent electrodes for trapping particulate matter. One such system is described in U.S. Pat. No. 4,594,138, issued Jun. 10, 1986 to Thompson. The Thompson filter utilizes electrostatic filtration in a cylindrical filter housing having an external electrical bus for connecting the electrodes.
A problem with prior electrostatic filters, such as the Thompson filter, is that over time the trapped particles within the porous filter media (typically constructed of paper, foam, or other suitable material) clog and diminish the efficiency of the filter, or clog and stop filtration altogether. When the efficiency of such prior filters becomes sufficiently low, it typically becomes necessary to shut down the entire filtration system, disassemble the filter device, remove the filtering media, install new filtering media, and then reassemble the device.
Another problem with such prior devices is that the efficiency of the filtering media cannot be easily determined without disassembling the filtering device and visually inspecting the filtering media.
Another problem with such prior electrostatic filtering devices is that they typically are complicated in design and, correspondingly, are difficult to manufacture and maintain.